Scalable Single-Ion Selective Mixed Matrix Ion Exchange Membranes for Direct Lithium Extraction

Electrodialysis with ion exchange (IEX) membranes offers a sustainable pathway for recovering critical minerals such as lithium from brines. However, existing membranes cannot efficiently separate lithium ions (Li⁺) from sodium ions (Na⁺), exhibiting a low Li⁺/Na⁺ selectivity. Here, we report a LiMn₂O₄-incorporated mixed matrix (MM) IEX membrane based on a quaternized poly(oxy-2,6-dimethyl-1,4-phenylene) matrix that achieves effectively near-infinite Li⁺/Na⁺ selectivity (Na⁺ was below the instrument detection limit) under electrodialysis by combining Donnan exclusion and size exclusion for competing cations with a percolating Li⁺-conducting network. Structural analyses of the MM membranes confirm a homogeneous LiMn₂O₄ distribution in the polymer matrix, spinel-phase retention, and superior thermal stability. Higher annealing temperatures reduced membrane water uptake/swelling and improved Li⁺/Na⁺ selectivity in diffusion dialysis; although in high concentration Na⁺ solutions with low(er) Li⁺ concentrations, the flux of Na⁺ exceeded that of Li⁺. In electrodialysis mode, there was no Na⁺ flux, while the Li⁺ flux reached as high as 46 ± 3 mmol·h^-1·m^-2 for Li⁺/Na⁺ ratios of 1 and 4 ± 3 mmol·h^-1·m^-2 for a Li⁺/Na⁺ ratio of 80. Hence, in cases of electrically driven ion transport, the MM membranes produced nearly infinite Li⁺ selectivity over Na⁺. No Mn leaching or membrane degradation was observed after repeated electrodialysis operating cycles, as confirmed by ICP and elemental mapping, suggesting good membrane stability. Given the global push toward electrification and the surging demand for lithium, these nearly infinitely Li-selective MM membranes address a critical need for a cost-effective, energy-efficient, and low-water-use method to separate Li⁺ from Na⁺-rich brines.